Resonant energy-conversion systems, despite of their advantages, such as sinusoidal currents, soft switching capability, wide operating frequency range, etc., are relatively slowly superseding the classical solutions based on hard switching. The reason is that in a resonant circuit the peak current values are substantially exceeding the maximum load current. Therefore, the reactance elements, both the capacitors and inductors, shall be designed to store relatively large amounts of energy. This problem can be solved by increasing both the weight and dimensions of reactance elements. However, such approach is not economically viable, since it entails additional costs and, consequently, a higher price. A further unfavourable effect is the decrease in energy efficiency, because the increase in the inductive elements dimensions in resonant energy-conversion systems results in considerable losses in windings, particularly at frequencies above 100 kHz. Also increasing the ferromagnetic core dimensions, while maintaining a constant rms value of the magnetic flux density is the reason that losses increase linearly with the core volume. Recently, due to rising electricity prices and legislative measures aimed at limitation of electric power consumption and its rational utilization, the energy efficiency becomes the crucial parameter influencing the potential success of the proposed solution.
The U.S. Pat. No. 5,886,516 presents an integrated multi-winding magnetic element intended for operation in a series resonant converter, in which on a single “UU” gapped magnetic core there are located two windings of an isolation transformer and two additional windings constituting two inductive elements of the resonant circuit. This assembly constitutes a resonant circuit consisting of three inductances, two capacitances and the isolation transformer.
An integrated-magnetic apparatus is known from the U.S. Pat. No. 5,726,615 comprising three ferromagnetic pot cores, two of which have central core-columns carrying two flat windings placed around these columns. These two inductive elements constitute a transformer. The third ferromagnetic pot core has a shorter central core-column around which a flat winding is placed. The third core-piece located adjacent to a flat exterior surface of the transformer allows to form the third inductive element. The third inductive element is partially coupled magnetically through an air gap to the other windings and is phased to have the magnetic induction in the same direction as the magnetic induction in the un-gapped magnetic circuit.
The U.S. Pat. No. 7,525,406 presents a structure that contains a plurality of coupled and non-coupled inductive elements and at least one closed magnetic circuit comprised of mutually contiguous magnetic elements having groves for current conductors in the X-axis and a perpendicular Y-axis. The current conductors located along the same axis exhibit mutual inductance but none between mutually orthogonal axes.
The Polish patent application No. 393133 presents a method for increasing the power transferred by an integrated inductor characterized by positioning an integrated inductor's windings orthogonally with respect to each other and the choice of induction elements values so that magnetic flux of the auxiliary magnetic circuit is transferred through at least a portion of the main magnetic circuit transferring the main magnetic flux while both magnetic induction vectors are oriented orthogonally with respect to each other, in addition both variable in time magnetic induction vectors are shifted with respect to each other in the time domain.
In the article “1 MHz-1 kW LLC Resonant Converter with Integrated Magnetics”, Zhang, Yanjun Xu, Dehong Mino, Kazuaki Sasagawa, Kiyoaki, Applied Power Electronics Conference, APEC 2007—Twenty Second Annual IEEE, Feb. 25, 2007-Mar. 1, 2007, pp. 955-961, there is described an integrated magnetic module in which the region of magnetic induction compensation is restricted to a small portion of the magnetic core volume. Moreover, in this element there occurs a problem of large resonant induction values with respect to the transformer induction value and also a relatively large effect of increasing the resistance of copper windings being in magnetic field from air gaps in magnetic circuits.
The article “Planar Integrated Magnetics Design in Wide Input Range DC-DC Converter for Fuel Cell Application”, Ziwei Ouyang, Zhe Zhang, Ole C. Thomsen, Michael A. E. Andersen, Ole Poulsen, Thomas Björklund, Energy Conversion Congress and Exposition (ECCE), 2010 IEEE: 12-16 Sep. 2010, pp. 4611-4618, also describes an integrated magnetic module in which the region of magnetic induction compensation is restricted to a small portion of the magnetic core volume. In this solution, a so-called hot spot occurs, where magnetic induction vectors produced by inductive elements of integrated magnetic circuits are summing up.
The above examples illustrate integrated reactances intended for use in resonant DC/DC converters. Nevertheless, said integrated reactances do not fully utilize the multi-winding inductor as an output transformer in resonant energy-conversion systems, and therefore, a reduction of thermal losses in inductive elements of the resonant circuit.
It would be, therefore, advisable to develop an integrated reactance element, characterized by reduced thermal losses in its resonant circuit inductive elements, and suitable for use in resonant DC/DC converters.